At one client, they were using a home-grown issue tracking system called Bob, after Bob the Builder. It even had an image of Bob the Builder in the application itself.

As a little joke, I built in a timer that at random times replaced the image of Bob with one where one eye was closed, wich made it look like he was blinking every now and then. It took a week and a half before someone noticed. I had a hard time not to laugh and to keep my face straight, while asking: "what do you mean, Bob is blinking? Are you serious?"